Economic feasibility of methods for secondary and tertiary recovery of petroleum often depends on accurate measurement of the quantity and location of reservoir water in a formation after previous recovery processes have been completed. Such measurements are desirably carried out in "old" wells, i.e., in wells used to produce the formation. Reasons: (i) accuracy is increased; and (ii) costs are decreased; the process of drilling a new well displaces some fraction of the formation fluids originally in the formation away from the hole, and it is desirable to evaluate the potential recovery from a reservoir without incurring the expenses of drilling a new well.
In my U.S. Pat. No. 3,817,328 for "Neutrons Absorption and Oxygen Log for Measuring Oil Content of Formations", June 18, 1974, assigned to the assignee of this application, I describe a method for accurately determining the oil content of a reservoir containing both mobile oil and a significant gas saturation. The first step was the recording of the response of both thermal-neutron-decay-time log and a neutron-activated-oxygen log to a formation penetrated by a well bore. A purposeful change was then made in the oil saturation in a given region of the formation surrounding the well bore by injecting fluid under sufficient pressure to displace the connate fluids. The change should remove substantially all the oil or remove as much oil as could be displaced by a proposed flooding technique. The combination of the thermal-neutron-decay-time log and the oxygen log was then run again to record the response of the same given region. The difference in the oil content around the well bore was determined from the differences between the two sets of logs.
My method may be somewhat limited, however, by the requirement that the oxygen activation log be calibrated at least to the extent that changes in log readings be proportional to changes in the oxygen content of the reservoir fluids with a predetermined single constant or proportionality. The responses from logging tools currently available are unduly influenced--in some applications--by the pipe, cement and liquids in the well bore bore; experience has shown that calibration valid at all depths in the well is difficult (if not impossible) to achieve in such applications.
Also many oil reservoirs, e.g., the SACROC field in the West Texas region of the United States, present additional problems in attempting to measure water saturation in and around existing wells; while the production brine may be saline enough in such reservoirs that conventional methods could work, the brine in the formation is often significantly different between producing intervals. Reason: it has been common practice for secondary recovery purposes to inject brine with salinity different from that in the virgin reservoir. At a later time when an accurate measurement of formation water is desired, the salinity of the water in the formation--a function of the degree of flushing of the injected water--is different from interval to interval within the reservoir.
Conventional techniques have also provided several logs which serve as useful fluid indicators (including formation water) which could be run through casing, and which are based on measurements reflecting the abundance of elements other than chlorine. These include logs designed to reflect the activation of oxygen nuclei as well as logs designed to measure the inelastic scattering of nuclei by carbon and oxygen. Oxygen abundance can also be gained from background correction applied to some versions of pulsed-neutron-capture logs. But the recorded signals from each of the logging devices required to carry out the above-mentioned methods only indicate the presence of formation fluids such as water, rather than its abundance, mainly due to difficulty in separating responses due to oil from those associated with the formation water.
In summary, the existing prior art methods of which I am aware are not quantitative enough to provide accurate determination of the frictional content of water in a reservoir rock containing oil, water and gas, especially if the formation brine is nonsaline and/or has been varied by previous waterfloods.